The present invention relates to a mixing device, especially designed to improve the mixing of two fluids such as exhaust gases from a diesel engine and an aqueous solution of urea.
Exhaust gas formed in the combustion of fuel in an internal combustion engine contains a proportion of undesirable substances such as nitrogen oxides (NOx), carbon monoxide (CO), un-burnt hydrocarbons (HC), soot, etc. . . .
To reduce air pollution, vehicles are therefore equipped with various systems that deal with undesirable substances in exhaust gas.
A common exhaust gas treatment is a so called selective catalytic reduction (SCR). Exhaust gas wherein ammonia is added as a reducer is treated in a specific catalytic converter where nitrogen oxides are converted into water and nitrogen which are both non toxic substances. Ammonia is introduced in the form of urea in an aqueous solution from which ammonia is obtained through hydrolysis. Urea is usually nebulised in the exhaust gas upstream of the catalytic converter. To this end, a urea injection nozzle is fitted on the exhaust line upstream the catalytic converter.
A problem with this type of exhaust gas treatment is that, before it has transformed into ammonia, urea can crystallize. In concrete terms, the aqueous solution of urea which is sprayed through the nozzle inside the exhaust pipe, according to a direction which is angled with respect to the exhaust gases flow direction, tends to form a solid deposit on the exhaust gas pipe wall, on the internal side thereof, opposite of the injection point. The consequence is that the cross section of the exhaust pipe is progressively reduced, which makes the engine efficiency decrease and which can seriously impair the engine operation in the long term.
It therefore appears that there is room for improvement in the treatment of exhaust gases of industrial vehicles.
It is desirable to provide a mixing system between two fluids which ensures a satisfactory mixing of said fluids into a pipe but prevents the injected fluid from forming a deposit onto the pipe surface, especially opposite the injection point.
An aspect of the invention concerns a mixing system comprising a pipe in which a first fluid can flow in a flow direction; a nozzle designed to inject a second fluid inside the pipe from an injection inlet arranged in the pipe wall, according to an injection direction.
Said mixing system further comprises a mixing device positioned inside the pipe upstream from the injection inlet; said mixing device has, at least, a first portion proximal to the injection inlet and a second portion distal to the injection inlet, said first and second portions being designed so that the velocity of the first fluid is higher downstream from the mixing device second portion than downstream from the mixing device first portion.
Thus, the invention provides a mixing device making it possible to obtain an asymmetrical velocity profile downstream from said device (as seen in the pipe median plane containing the nozzle axis). As a consequence, opposite the injection inlet, the first fluid flows quickly and drags the second fluid, making it deviate downstream with respect to its injection direction, thereby preventing said second fluid from hitting the pipe and making a deposit on it.
This result is obtained without impairing the mixing efficiency, since (i) the mixing device is designed to generate turbulence, thereby contributing to a better mixing of the fluids and (ii) the first fluid velocity downstream from said mixing device is not uniformly high, which would not provide enough time for both fluids to efficiently mix before they reach the pipe outlet.
It has to be noted that the mixing device is located close to the injection inlet, so that the velocity profile is substantially not altered when the first and second fluids meet.
Preferably, said first and second portions have several passageways for the first fluid, said passageways having a greater combined cross section in the second portion compared with the first portion. This arrangement ensures that an asymmetrical velocity profile can be obtained downstream from said mixing device.
Preferably, said first portion of the mixing device generates more turbulence in the first fluid flow than the second portion.
At least said first portion of the mixing device may comprise flow diverting members.
In an implementation of the invention, said mixing device can consist of or comprise said first and second portions, said portions being shaped as half discs and being located on both sides of the pipe diameter plane which is orthogonal to the pipe median plane containing the nozzle axis.
Alternatively, said mixing device can comprise at least one intermediate portion located between said first and second portions, said intermediate portion being designed so that the first fluid velocity downstream from the mixing device intermediate portion is higher than downstream from the mixing device first portion and lesser than downstream from the mixing device second portion. With this arrangement, a progressively increasing velocity of the first fluid can be achieved.
For example, the passageways can have a substantially constant size and can be arranged at substantially regular intervals in at least one or each of the mixing device portions.
In an advantageous way, the mixing device may comprise several successive intermediate portions arranged as substantially parallel strips between said first and second portions, the passageways having an increasing combined cross-section from the first portion towards the second portion.
The mixing system may comprise several portions arranged as angular sectors as in a pie-chart.
In order to easily fasten the mixing device inside the pipe, it is envisaged that the mixing device comprise a substantially cylindrical sleeve designed to be fastened axially inside the pipe, said at least first and second portions being arranged inside said sleeve.
According to a first embodiment of the invention, said mixing device comprises a central axis substantially identical to the pipe axis and blades (preferably helical blades) extending radially from said central axis, the spacing between successive blades being greater in the second portion compared with the first portion. This type of device is able to create a swirl movement of the fluids, thereby ensuring a particularly efficient mixing.
According to a second embodiment of the invention, said mixing device comprises a plate equipped with a plurality of holes, the combined cross-section of the holes being greater in the second portion compared with the first portion.
According to a third embodiment of the invention, said mixing device comprises, in each of said at least first and second portions, a plurality of fins angled with respect to the flow direction, said fins generating more turbulence in the first portion than in the second portion. This type of device is also able to generate turbulence helping the mixing of both fluids.
A specific application of the invention is the treatment of NOx in exhaust gases. In that case, said pipe is an exhaust pipe of a diesel engine and said second fluid is an aqueous solution of urea.
The invention makes it possible to obtain a satisfactory mixing between exhaust gases and urea and then, further downstream, between NOx and ammonia when urea has broken down. Therefore, it is possible to effectively reduce the NOx compounds and to achieve considerably lower NOx emissions. At the same time, the invention effectively prevents urea that has not broken down into ammonia yet from making a deposit on the pipe opposite its injection pipe, thereby increasing the service life of said exhaust pipe.
These and other advantages will become apparent upon reading the following description in view of the drawing attached hereto representing, as non-limiting examples, embodiments of a vehicle according to the invention.